As surface-treated steel sheets for use in cans, tin-plated steel sheets, heretofore called “tinplates,” have been widely used. In general, the tin-plated steel sheets as described above are immersed in an aqueous solution containing a hexavalent chromium compound, such as bichromic acid, or are electrolyzed in the above solution or are coated therewith to form chromate layers on the plated surfaces of the steel sheets. By the formation of chromate layers on the steel-sheet surfaces, oxidation of the tin-plated surfaces can be prevented during long-term storage, and degradation in appearance (yellowing) can be suppressed. In addition, when paint is applied to the tin-plated steel sheet before use, since the growth of a tin oxide layer is suppressed, cohesive failure of the tin oxide layer is prevented, and hence adhesion of the paint is ensured.
However, when the chromate coating is formed on the tin-plated steel sheet surface, since an aqueous solution containing a hexavalent chromium oxide is used as described above, to secure safety of working environment and to carry out an effluent treatment, a considerable cost is required. Furthermore, in case that a chromate processing solution unfortunately leaks out by accident or the like, it may probably cause very serious damage to the environment.
As described above, because of recent trend toward environmental conservation, movement for restriction of the use of chromium has proceeded in various fields, and a chemical conversion treatment containing no chromium has been increasingly required even for the tin-plated steel sheets.
According to the above current situation, various chemical conversion techniques instead of the chromate treatment have been proposed for tin-plated steel sheets for use in cans. For example, in Japanese Examined Patent Application Publication No. 55-24516, a surface treatment method for a tin-plated steel sheet has been disclosed which forms a chemical conversion coating containing no Cr on a tin-plated steel sheet by direct current electrolysis performed in a phosphoric acid-based solution using the tin-plated steel sheet as a cathode.
In Japanese Examined Patent Application Publication No. 1-32308, an electroplated tinplate for use in seamless cans has been disclosed in which a chemical conversion coating which contains P with or without Al and no Cr is provided on a tin plating layer surface.
In addition, in Japanese Examined Patent Application Publication No. 58-41352, a chemical conversion solution for treating metal surfaces, which contains phosphate ions, at least one type of chlorate and bromate, and tin ions, and which has a pH of 3 to 6, has been disclosed.
However, in view of suppression of performance degradation, such as degradation in appearance (yellowing phenomenon) and degradation in paint adhesion, caused by the growth of a tin oxide layer on a surface, it cannot be said that the chemical conversion coatings disclosed by the above conventional techniques have sufficient performance equivalent to that of a chromate coating obtained by a conventional solution containing bichromic acid.
In addition, a tin-plated steel sheet which is processed by a current chromate treatment is generally manufactured at a high speed, such as 300 m/min or more, and hence it has a high productivity. Accordingly, to replace the tin-plated steel sheet which is processed by a chromate treatment with a new chemical conversion treatment, the new chemical conversion treatment must be performed at a high speed at least equivalent to or more than that of the current process. As a rough index at which a chemical conversion treatment is performed at a high speed of 300 m/min or more, a chemical conversion time is preferably set to approximately 1 second. When the chemical conversion treatment is completed within 1 second, a treatment at a speed of 300 m/min can be performed, for example, by using one relatively small and vertical-shaped tank having an effective depth of approximately 2.5 m. However, as the treatment time is increased, the size of the treatment tank or the number thereof must be increased in order to ensure a passing time. As a result, equipment cost and maintenance cost thereof are both unfavorably increased.
It could therefore be advantageous to provide a tin-plated steel sheet and a manufacturing method thereof, the tin-plated steel sheet having a phosphoric acid-based chemical conversion coating instead of a conventional chromate coating, which can suppress the degradation in performance caused by the growth of a tin oxide layer on a surface. Furthermore, it could also be advantageous to provide a method which can manufacture the above mentioned steel sheet while high speed and high stability equivalent to those of a conventional chromate treatment process are maintained.